im in master difficulty, if it matters. anyways i always us shock spells for thier speed, but im uncertain if frost is better for slow of fire for cost reductions?

I typically use fire for most enemies, but electric for mages (because it also damages magicka), and dragons (because it's MUCH easier to hit moving targets with). Also, anything that's immune to fire, like flame atronachs (besides, they move pretty fast too, sometimes). I've thought of using frost for melee enemies, since it damages stamina, but I think fire kills them faster, and killing them faster seems to prevent more damage than draining their stamina.

That's just my experience and opinion.

Frost-spells is better for slowing them down and draining Stamina. Very good against fire atronarchs.
Fire is good for high-damage
Lightning is good against magicka-users since it drains their magick power.

I have played with like 5 or 7 mage characters, all of them with destruction. And now I love shock spells. Fire is good, but you wont be using too much the master spell, and the enemies can dodge the balls you throw at them. One of those mages was meant to be a frost one and I dont recomend it, all the frost spells have almost the same speed of fire, and the slowing enemies is not so useful as I though.

And the most important thing, a lot of enemies have a 50% frost resistance : Frost fragons, all draugs and undeads, vampires, frost throlls, frost wolfs and bears, all nord NPCs

If you wanna try something apart from shock I would recomend you fire definitely!

Shock is 20% more expensive than Fire, and does 10% less damage.

The cost is probably most noticeable in early game. Later on, as you become a stronger mage you won't even feel the cost difference.

There are some things that are resistant to Fire. Dunmers and Fire type Dragons.

I find it much easier to hit things with Incinerate than Thunderbolt in most situations. Fire spell shots move the slowest, but the projectile size is much bigger. Also, Fire spells have very very long range. You can fire your shots when you can barely see them. You can't do that with Thunderbolt.

Frost spells are very good against Dunmers and Giants. The slow effect is very cool and a lot of fun. But Frost Spells are quite weak against Nords and Draugrs. But sometimes when I feel safe, I use weaker, more expensive spells intentionally to level up my magic skills faster.

I use all 3, but mostly Fire. Then Frost.

Use different strokes for different folks.